From Hard Drop - Tetris Wiki

Hold piece is a feature of most newer Tetrisgames starting with The New Tetris.
The top left of the tetrion has a window called the hold space that displays a tetromino.
At any time starting when a tetromino enters the playfield until it locks, the player can press the Hold button on the controller to move the active tetromino into the Hold Queue and move the tetromino that was in the hold space to the top of the playfield.
Be careful: if you swap the tetromino out from the Hold Queue, you have to use it right away—you can’t swap it back until the tetromino that was moved out of the Hold Queue locks! Using the Hold Queue correctly is one of the keys to becoming a great Tetris player—practice using it a lot and watch your scores go up and up.

What should you dowith this O tetromino?

Press the Hold button

Drop Z

O becomes availableafter Z locks

The Hold Queue was introduced as part of the development of Square Tetris mode (introduced in The New Tetris; also appears in Tetris Worlds), making it easier for the player to assemble squares.
Its inclusion in standard Tetris is controversial, as some critics have claimed that it makes the game too easy.[1][2][3][4]

Contents

Variants

Initial state of the hold box

In The New Tetris, the players start with a random piece in their hold box. In guideline games however, the Hold Queue is initially empty, and the first use of the hold causes the next piece to appear immediately.

Orientation

Most games rotate tetrominoes back to the starting orientation when moving them to the Hold Queue. Tetris Worlds, on the other hand, does not (at least on GBA). The rotation back to the starting orientation therefore must have been added to the Tetris Guideline between 2002 and 2006.

Both modes of TGM3 (Classic and World) include a feature called "initial hold system" or IHS, which allows the player to press and hold the Hold button during entry delay, and the tetromino will be swapped as soon as it enters the matrix, just as with IRS.

Lockjaw interprets a slightly different form of initial hold. At any time when no tetromino is falling (such as entry delay or line clear delay), the Hold button switches the hold piece with the next piece to come out.

Skip

Shadow mode in both Tetris Partygames feature "Skip" instead of the Hold Queue. It is activated like the Hold Queue, but the piece is simply skipped and not stored for later use. The number of skip use is limited; the player is given 10 skips in early levels, but later levels allow more. Unlike the hold, skip can be used twice or more in the row without locking a piece.

Tetris Blitz

In Tetris Blitz, the Hold Queue can be used twice or more in the row without locking the piece in between, effectively allowing the players to cancel the hold. The game also allows the players to buy the second Hold Queue via real money.